There is a wiki which aims to collate links to various resources and summaries of these materials on how to go about training. Please read this before posting to see if there is something useful already linked from there.

Beyond fitness and technique, learning to downclimb can really pay off in sticky situations outdoors. I've had my rope get stuck numerous times and been forced to solo then downclimb tricky terrain to retrieve it. It's an essential rockcraft skill.

Downclimbing can be super helpful. I am around your level and I have recently taken to (at the gym) downclimbing all the bouldering routes I can (usually V0-V3). I've found that downclimbing bouldering routes and then going up and down again a few times (I usually do 3-4 reps without rest both up and down) has made me a much stronger climber.

I love downclimbing, it can be both a great warm-up and a good way to end a climbing session. I'd recommend starting on something a grade or two easier than your normal warm-up if you downclimb as a warm-up though. Usually just go up then down, but if it's the end of a session it's fun to see how many laps you can get in. Belaying can be difficult at first though. You can keep one hand above the atc and one below without having to let go of the rope. Just sliding the hands.

Eccentric muscle contraction (which occurs during down-climbing) works a give muscle ~40% more than concentric contraction (occurs during normal climbing). It also forces you to be more aware of your foot placement as well as the positive aspects of any holds so that you don't fall off.